SUNDAY SPECIAL / Valley Briefing

Controlling Water Flow

It's not the Hoover Dam--it doesn't provide hydroelectric power to entire cities. But Sepulveda Dam's primary function is vitally important. As one of 19 dams in Los Angeles County's flood control system, it protects thousands of San Fernando Valley residents when winter flood waters stream down from higher elevations.

The dam dominates the 2,100-acre Sepulveda Dam Recreation Area, which also serves as a flood control basin. The Los Angeles River travels for about seven miles of concrete-lined channel before reaching the dam, which regulates the flow so that points downstream are not flooded. Although the Los Angeles River channel remains dry most of the year, once the rainy season officially begins Nov. 15, Sepulveda Dam is manned, water flow or not.

Drainage Area Map

Sepulveda Basin regulates runoff from a 152-square-mile area. Any water that doesn't soak into the ground in that area flows via flood control channels and the L.A. River to the dam, and eventually the ocean. The highest point in the watershed area is San Fernando Peak in the Santa Susana Mountains, at 3,741 feet. The lowest point is the channel below the dam facility control house--668 feet above sea level.

Opening the Floodgates

Water in the channel is measured using three types of gauges--precipitation, stream flow and water surface--and the data transmitted by telemetry to the Army Corps' central office downtown. Using this information and the dam keeper's visual observations, Army Corps officials determine which dams need to release water in order to prevent flooding.

How The Gates Work

1. Water flows from approach channel to dam. Using data from water gauges and dam keeper, Army Corps officials downtown calculate what gate setting should be and radio instructions to dam keeper.

2. Dam keeper operates the gates separately, using hydraulic lifts in steel cylinders inside control house. Each gate can be raised a maximum of nine feet.

Historical Data

* The Sepulveda Dam and reservoir project was completed in 1941, following a destructive flood in February and March of 1938. The process of lining the Los Angeles River with concrete occurred from the 1930s to the 1960s.

* The highest the water has ever risen at the dam was in 1980, when it came within 20 feet of the dam's top.

* In a 1980 flood, the peak inflow of water at the dam was 59,000 cubic feet per second (cfs). The channel capacity directly below the dam is 16,900 cfs.

* In the 1980 flood, water reached an elevation level of 705.1 feet in the spillway, about five feet short of the top of the gates. The reservoir behind the dam was filled to about two-thirds its 17,425-acre-feet capacity.

* During heavy rains, currents can blast through floodgates at up to 60 mph.

Facts About the Dam

* Height: 57 feet

* Length: 3 miles

* Construction: Earth fill with reinforced concrete outlets and control house

* One of 19 dams in L.A. County

* Safety: Dam is inspected periodically, and gates are raised and lowered twice a month. The entire plant is inspected annually and gets a complete safety check every five years.

The U.S. Army Corps of Engineers operates the dam system, coordinating water flow reports from dam keepers throughout the network. The Sepulveda Dam keeper is stationed at the control house and stays busy checking water gauges and inspecting outlet and spillway gates.

* Outlets: All eight outlets are kept open throughout the year

* Dam Crest: Dam is 57 feet high from top to bottom.

* Spillways: The spillway acts as a relief valve, preventing water from going over the dam embankment. Each spillway bay has a steel drum gate designed to float in a chamber located in the spillway crest.

Sources: U.S. Army Corps of Engineers; Times staff reports. Researched by JULIE SHEER / Los Angeles Times